1) Field of the Invention
The present invention relates generally to the control of systems for the heating or cooling of fluids, e.g., air or water. In particular, the present invention relates to provision of systems and techniques for variable operation of such systems.
2) Discussion of the Related Art
In the field of gas burner technology relating to burners such as may be used in furnaces, water heaters, boilers, and the like, it is desirable to control the operation of a burner beyond merely supplying gas and providing air for combustion at a fixed flow rate, and igniting the mixture. Numerous factors must be considered in the construction, placement and operating conditions for a gas burner.
Typically, variably controllable parts of a burner appliance may include the combustion fan also sometimes called the inducer fan, which creates a negative pressure in the combustion area to supply air to the combustion process and create draft to ensure removal of the products of combustion. Terminology in the art will sometimes distinguish a power burner which uses positive pressure, and an induced draft burner which uses negative pressure. A circulator fan may be used to variably control movement of the treated air, such as by blowing over the heat exchanger for the movement of heated air. “Fan”, “motor” and “blower” may sometimes be used interchangeably herein in referring to motor driven fans for air movement. Variable fuel valves are known in the art which can modulate, or vary, the supply of fuel to a burner. “Appliance” will be used herein in the sense of a hardware device such as a burner or condenser for heating or cooling, or a larger apparatus such as a furnace or air conditioning unit using such a burner or condenser.
In general it is true that a burner which operates closely to stoichiometric conditions is more efficient than one which is operating, for example, with a large amount of excess air. If the amount of fuel gas and combustion air are known, the actual combustion conditions, relative to stoichiometry, may be defined.
Problems faced by gas burners include performance variations caused by changes in airflow, such as due to fan/blower degradation and flue blockage. Variations in burner performance caused by the aforementioned conditions may result in excessive pollutant production, which in turn may be a health and safety hazard. Some prior art appliances provide a fixed air supply to a burner, and must, therefore, supply enough air to prevent excessive production of deleterious gases such as carbon monoxide and oxides of nitrogen under ideal operating conditions, and also provide a safety margin to account for incidences such as a blocked stack or an overfire condition (i.e., a significant increase in the firing rate above the rated value) within the appliance. Therefore, a standard appliance is typically designed with an excess air level significantly higher than would be required if changes in firing rate or airflow could be compensated for automatically. The additional safety margin of excess air may result in a significant reduction in appliance efficiency. Accordingly, it would be desirable to more closely control the fuel to air ratio to achieve greater efficiency.
An additional problem that gas burner equipped appliances, such as furnaces, face, is the effect that altitude has upon performance. At higher altitudes, burners receive air that is less dense, and accordingly, has less oxygen. Accordingly, for appliances that are not capable of modifying their operation in response to altitude, such apparatus must be derated for altitudes that are different than a “base” or nominal optimum operating altitude (e.g., sea level). For example, it is typical to derate an appliance, such as a furnace, at a rate of −4% per every 1000 feet of increased altitude. That means that for an appliance having a rating of X BTU/Hr at sea level, the rating may be X*(1-0.04) BTU/Hr at 1000 feet.
Gas burning appliance designs are known in which the supplies of fuel gas, primary combustion air and secondary combustion air (if such is applied) are capable of being physically controlled in finite increments to facilitate safe and efficient operation. However, with prior designs, this is typically achieved through the use of complex mechanical systems, such as a mechanical jackshaft. Known appliances may have the capability to modulate or vary fuel flow over a wide supply range, thus providing a wide range of heating capacity (firing rates) through a single appliance. However the known variable systems are presently very expensive. Modulating fuel capabilities may greatly increase a system's overall efficiency. Two stage systems, i.e., systems capable of operating at two firing rate levels, are available, but are limited in their scope and range of operation due to their inability to precisely control the fuel gas and air mixture at two levels only, and the need for a wide excess-air safety margin.
As stated, a continuously modulating appliance, to be efficient, may require close control of the fuel/air ratio. Though it is possible to directly measure the fuel and airflow rates independently and thereby determine the fuel and air mixture, such a detection system would require expensive sensor systems and be complex and possibly overly costly for most appliance applications of interest. A known system as taught in U.S. Pat. No. 5,971,745 may therefore be used.
Various other techniques or systems to increase the efficiency of an air treatment system have been proposed. Variable speed motors for blowers, fans, etc., for air movement have been used to a limited degree but they, alone, do not allow the appliance to vary its output since other components must also be varied to safely modulate a combustion appliance. Further, most commercially available variable speed motors are expensive.
It is also generally true that the more modulation and control capability placed into an appliance system, the greater the cost to supply and maintain sensing and control of that system to achieve the desired efficiency increases. However, the applicants do not believe that a control system for integrating all factors of a variable heating or cooling system has yet been presented which takes full advantage of the efficiencies to be gained from such systems while providing variable control at a reasonable cost and performance level.